1. Field of the Invention
The present invention relates to a magnetoresistive element for use in magnetic recording devices, oscillators, magnetoresistive random access memories (MRAMs), magnetic sensors and the like, and to a thin-film magnetic head, a head assembly, and a magnetic recording device that each include the magnetoresistive element.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in areal recording density, and thin-film magnetic heads of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head including an induction-type electromagnetic transducer for writing are stacked on a substrate.
Examples of MR elements include a giant magnetoresistive (GMR) element utilizing a giant magnetoresistive effect and a tunneling magnetoresistive (TMR) element utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
Spin-valve GMR elements and TMR elements each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer opposite from the spacer layer. The free layer is a ferromagnetic layer whose direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose direction of magnetization is pinned. The antiferromagnetic layer is a layer that pins the direction of magnetization of the pinned layer by means of exchange coupling with the pinned layer. For spin-valve GMR elements, the spacer layer is a nonmagnetic conductive layer. For TMR elements, the spacer layer is a tunnel barrier layer. The tunnel barrier layer is typically an insulating layer made of an insulating material such as aluminum oxide or magnesium oxide.
Examples of the GMR elements include one having a current-in-plane (CIP) structure in which a current for magnetic signal detection (hereinafter referred to as sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element, and one having a current-perpendicular-to-plane (CPP) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element. Hereinafter, a GMR element that has the CPP structure will be referred to as a CPP-GMR element, and a GMR element that has the CIP structure will be referred to as a CIP-GMR element. TMR elements also have the CPP structure.
In recent years, with an increase in recording density, there have been increasing demands for a reduction in track width of the read head. A reduction in track width of the read head is achievable by reducing the width of the MR element. As the width of the MR element is reduced, the length of the MR element in a direction perpendicular to the medium facing surface, which is the surface of the thin-film magnetic head to face the recording medium, is also reduced. As a result, the top and bottom surfaces of the MR element are reduced in area.
The read head using a CIP-GMR element includes shield gap films for separating the CIP-GMR element from respective shield layers that are disposed over and below the CIP-GMR element. The heat dissipation efficiency therefore drops if the top and bottom surfaces of the CIP-GMR element are reduced in area. Such a read head has the problem that the operating current is limited in order to ensure reliability.
In contrast, the read head using a CPP-GMR element needs no shield gap film, and the top and bottom surfaces of the CPP-GMR element are in contact with respective electrode layers. The electrode layers may also serve as shield layers. Such a read head has a high heat dissipation efficiency since the top and bottom surfaces of the CPP-GMR element are in contact with the respective electrode layers. This makes it possible to increase the operating current of the read head. In such a read head, the smaller the areas of the top and bottom surfaces of the CPP-GMR element, the higher the resistance of the element and the greater the magnetoresistance change amount of the element. The CPP-GMR element is thus suited to reduce the track width.
A typical CPP-GMR element, however, shows a small magnetoresistance change amount because the spacer layer, i.e., a nonmagnetic conductive layer, has low resistance. Accordingly, there is a problem that it is not possible to obtain a sufficiently high value for the magnetoresistance change ratio, i.e., the ratio of the magnetoresistance change to the resistance of the element (hereinafter referred to as MR change ratio).
On the other hand, TMR elements have the following problem. TMR elements have high resistance since their spacer layer has high resistance contrary to the foregoing typical CPP-GMR element. There is a need for magnetic disk drives that have an improved data transfer rate as well as improved recording density. Favorable high-frequency response is thus required of the read head. The high resistance of the TMR element, however, increases the stray capacitance occurring in the TMR element and in the circuits connected thereto, which degrades the high-frequency response of the read head.
Under the circumstances, various proposals have been made, as described below, for making the resistance of the spacer layer of the CPP-GMR element and the resistance of the element appropriate in value.
U.S. Pat. No. 7,072,153 describes a CPP-GMR element of current confined type. The CPP-GMR element includes: a magnetization pinned layer whose direction of magnetization is pinned; a magnetization free layer whose direction of magnetization changes in response to an external magnetic field; and an intermediate layer that is provided between the magnetization pinned layer and the magnetization free layer. The intermediate layer includes a first layer (oxide intermediate layer) that is made of an oxide having a region of relatively high resistance and a region of relatively low resistance. The sense current flows preferentially through the region of relatively low resistance when passing the first layer.
U.S. Pat. No. 7,218,483 describes a CPP-GMR element as follows. The CPP-GMR element includes: a magnetization pinned layer whose direction of magnetization is pinned; a magnetization free layer whose direction of magnetization changes in response to an external magnetic field; a nonmagnetic metal intermediate layer that is provided between the magnetization pinned layer and the magnetization free layer; and a resistance adjustment layer that is provided between the magnetization pinned layer and the magnetization free layer and is made of a material containing conduction carriers no more than 1022/cm3. The document describes that a semiconductor or a semimetal is desirable as the material of the resistance adjustment layer, and lists ZnO as an example of the semiconductor.
U.S. Patent Application Publication No. 2008/0062557 A1 and U.S. Patent Application Publication No. 2009/0002893 A1 each describe a CPP-GMR element whose spacer layer includes a layer formed of an oxide semiconductor such as ZnO.
U.S. Patent Application Publication No. 2009/0086383 A1 describes a CPP-GMR element whose spacer layer includes a layer formed of an oxide of Zn, Ga, or the like.
For the CPP-GMR elements, providing a spacer layer that includes a layer formed of an oxide semiconductor is considered to be useful in making the resistances of the spacer layer and the element appropriate in value. The inventors of this application actually fabricated CPP-GMR elements having a spacer layer including a layer formed of an oxide semiconductor by using various types of oxide semiconductors, and examined the elements for characteristics. The results revealed that it is difficult to make the element resistance appropriate in value and make the MR change ratio sufficiently high if the spacer layer includes only a layer of a single type of oxide semiconductor as the layer formed of an oxide semiconductor.